1. Field of the Invention
The present invention relates generally to an image sensor, more particularly, to an image sensor having plurality of pixels arranged in rows and columns, for use in such an electronic camera, an image reader or a facsimile, with a stabilized black level and low power consumption.
2. Description of the Related Art
In an image sensor, each light receiving element outputs a signal having an integrated light component and an integrated dark current component. This dark current has a strong temperature dependency that the current becomes about twice with rise of 9° C.
In order to remove the integrated dark current component from the signal to obtain true integrated light signal, in the prior art, the peripheral region of a pixel array was covered with a light shielding film to form an optical black pixel region, and an integrated dark current signal was read out from the optical black pixels in a blanking period to obtain the average voltage Vd of the integrated dark current signals in a black clamp circuit as an offset value (a black clamp level). The average Vd was subtracted from each pixel signal Vs in the black clamp circuit in a period of reading out from an effective pixel region.
There are two kind of black clamps, one is a line black clamp performed in each horizontal scanning, the other is a frame black clamp performed in each frame, and one of them is adopted.
FIG. 19 shows an integrated dark current signal read out from an optical black pixel region in a horizontal blanking period. The signal is not constant according to characteristics of each pixel.
Especially, when a defective pixel exists, an integrated dark current signal suddenly changes as shown in FIG. 19, therefore an offset value becomes incorrect in a black clamp circuit having a integration capacitor for obtaining an average value, which causes lateral stripe noise. If the width of the optical black pixel region is wide in order to prevent the noise, the offset component could not be obtained in a horizontal blanking period in an image sensor having quite many pixels, or a chip area increases with the result of higher cost.
On the other hand, in an image sensor adopted in portable equipment, reduction in power consumption has been required.
There are following methods for reduction in power consumption of a semiconductor chip:
(1) to stop an operation clock in a period where no operation is required; and
(2) to reduce a clock frequency by a divider, for example, to a half, a quarter and one-eighths thereof.
However, the chip of an image sensor includes an analogue circuit and a digital circuit, power consumption of the analog circuit is larger than that of the digital circuit with a large ratio, and power consumption of an image sensor for taking a moving picture cannot decrease to a great extent by such methods.
When a power source for the analog circuit is on/off controlled, the influence of off appears after one frame, therefore the power source cannot be simply turned off.